This invention relates generally to implantable cardiac therapy devices and ore particularly to the forecasting of the impending onset of a cardiac arrhythmia.
Sudden cardiac death (SCD) presents a public health challenge in that often the only indication a patient is at risk appears when the patient succumbs, without warning, to an episode. This is true in part because one major form of SCD is cardiac arrhythmia, typically either ventricular tachycardia that degenerates into ventricular fibrillation, or in some cases spontaneous fibrillation itself. As the underlying physiological causes of cardiac arrhythmias are not fully understood, they cannot be accurately predicted.
Remarkably many victims survive episodes of SCD, either because their arrhythmias spontaneously terminate or because external defibrillation is rapidly implemented. These patients form the bulk of the candidates for implantable cardioverter defibrillators (ICDs). Historically, the hallmark indication for ICD implantation was prior survival of the patient from a confirmed episode of non-sustained tachycardia or fibrillation (in the case of ventricular-based SCD), along with elimination of other possible causes of the arrhythmias. Recently, the size of the target population has also increased due to prophylactic use of ICDs for treatment of SCD based on other risk indicators.
But, in either case, current ICD technology relies on identification and confirmation of an ongoing arrhythmia, primarily based on the sensed heart rate, as opposed to prediction of onset of arrhythmia based on non-arrhythmic conditions, i.e. heart rates below a tachycardia threshold. Previous research into prediction of arrhythmias is abundant but generally directed at risk stratification, that is, statistical prediction of risk over the long-term in a large segment of the population, as opposed to short-term prediction based on the real-time symptoms of a specific individual patient.
In U.S. Pat. No. 5,042,497 (Shapland), incorporated herein by reference, a system for predicting arrhythmias based on sensing neural activity of a patient is disclosed. However, this patent appears to examine only increased arrhythmic risk due to changes in neural tone and provides limited insight into how such a system would be implemented. It would be desirable to provide a predictive method that could be implemented using existing ICD technology and be based on morphology changes of any origin.
The invention is a method and apparatus for forecasting impending onset of arrhythmia based upon analysis of real-time intact (i,e, non-decomposed) intracardiac electrograms. Real-time changes in the sensed intracardiac electrogram such as changes in morphology that are precursors of arrhythmia are identified in non-arrhythmic complexes of the intracardiac electrogram. The prediction analysis may be performed on a continuous or periodic basis. Other parameters such as heart rate may be used as weighting factors in the analysis. One embodiment of the invention relies on a decrease over time in the degree of correlation between a normal sinus waveform morphology template and the waveform morphology of complexes in a moving window of the intracardiac electrogram to determine whether to forecast an impending arrhythmia. A second embodiment relies on an increase in the degree of correlation over time between a pre-arrhythmic waveform morphology and the waveform morphology of complexes in a moving window of the non-arrhythmic intracardiac electrogram to determine whether to forecast an impending arrhythmia.